1. Field of the Invention
The present invention relates to a class D amplifier, and more particularly to a class D amplifier having a correction circuit.
2. Description of the Background Art
Class D amplification has conventionally been adopted as a method of enabling device miniaturization by performing power amplification on audio signals with high efficiency and low power loss. A class D amplifier is known in which a digitized audio signal is directly converted to a pulse width modulated signal and is guided to a power switch. The power switch usually includes a switching device connected to a constant voltage power supply and a switching device connected to a ground (or negative power supply).
Further known is a method for reducing rounding errors caused by a requantizer required for the PWM (pulse width modulation) conversion by means of delta-sigma modulation, which allows a PWM (pulse width modulated) signal of high accuracy to be obtained. Such PWM signal is output from the power switch with high accuracy, so that an audio signal of high quality can be output from the class D amplifier (cf. Japanese Patent Application Laid-Open Nos. 11-261347 (1999) and 2001-292040).
However, the above-described method actually causes a problem in that the use of an ideal constant-voltage power supply for the power supply of the power switch is generally difficult to realize on cost grounds and a problem in that consumption power in the constant-voltage power supply is increased, which results in loss of inherent advantages of the class D amplifier. In view of these problems, compromises have been made in many cases, though not sufficient, which only suppresses an alternating component of audio frequency which particularly comes into question because of an LC filter.
Further, according to the above-described method, the power switching devices forming the power switch each have a finite delay time for turn-on and turn-off. Therefore, it is generally difficult to turn ON one of the switching device connected to the constant voltage power supply and the switching device connected to the ground, and turn OFF the other one with the same timing. Thus, it has been required to set a dead time after one of the devices is turned OFF almost completely and until the other one is turned ON.
The above-mentioned fluctuations in the supply voltage directly appear as fluctuations in the amplitude of an output signal from the power switch, causing distortion in an audio signal output from the amplifier.
Further, distortion in an output signal from the power switch resulting from the dead time setting also causes distortion in an audio signal output from the amplifier.
As measures for the aforementioned problems, a correction system according to a conventional technique is known (cf. National Publication of Translation No. 2001-51739; e.g., FIGS. 3-8). The conventional technique will specifically be described hereinbelow in reference to drawings showing the configuration.
FIG. 32 is a block diagram illustrating the configuration of a class D amplifier having a conventional correction system.
In FIG. 32, a pulse modulator 100, a correction unit 102, a power switch 103 and an LPF (low pass filter) 104 are connected in series to each other. Error handler 101 is connected between nodes N100 and N101 in parallel to the correction unit 102 and power switch 103 and has its output connected to the correction unit 102.
In the class D amplifier having the correction system configured as above described, the pulse modulator 100 generates a binary pulse modulated signal Vr by modulating an audio signal.
The power switch 103 performs power amplification by switching between a constant-voltage power source and the ground in accordance with a value of a correction signal Vc which is a binary pulse signal transmitted through the correction unit 102, enabling power supply to a load connected to the output of the amplifier. Here, the power switch 103 has a factor that causes distortion in an audio signal (hereinafter referred to as distortion factor) such as fluctuations in supply voltage and dead time setting for operations of the switching devices.
The error handler 101 detects deformation of an output signal generated by the power switch 103, and more specifically, detects an error contained in a feedback signal Vs output from the power switch 103 with reference to the pulse modulated signal Vr output from the pulse modulator 100, thereby generating and outputting an error signal Ve corresponding to the error.
The correction unit 102 corrects the pulse modulated signal Vr input from the pulse modulator 100 by changing its width in accordance with the error signal Ve from the error handler 101, thereby performing control so as to reduce the error signal Ve from the error handler 101.
The internal configuration of the correction unit 102 will specifically be described hereinbelow.
FIG. 33 is a block diagram illustrating the internal configuration of the correction unit 102. In FIG. 33, an integrator 200, an amplitude limiter 201 and a “−” terminal of a comparator 202 are connected in series to each other. The comparator 202 has its “+” terminal connected to the output part of the error handler 101 and its output part connected to the input of the power switch 103. The integrator 200 has its input connected to the output of the pulse modulator 100.
Next, operations of the respective components of the correction unit 102 will be described referring to FIG. 34 which illustrates signal waveforms at respective points in the correction unit 102.
In FIG. 34, reference numeral 210 represents a waveform of the pulse modulated signal Vr input to the integrator 200, and 211 represents a trapezoidal waveform of an input signal Vi input to the “−” terminal of the comparator 202 which is obtained from the pulse modulated signal Vr converted while passing through the integrator 200 and amplitude limiter 201. By the action of the integrator 200, the falling edge and rising edge of the trapezoidal waveform 211 are inclined at a certain angle. The amplitude of the trapezoidal waveform 211 is limited within a certain range by the action of the amplitude limiter 201.
The reference numerals 212 and 213 each represent a waveform of the error signal Ve output from the error handler 101 and input to the “+” terminal of the comparator 202, and 214 and 215 each represent a waveform of the correction signal Vc generated in and output from the comparator 202 by comparing the input signal Vi and error signal Ve input to the comparator 202.
Here, the waveforms 212 and 213 are derived from error signals Ve having different values from each other. The waveform 214 is derived from the correction signal Vc generated in the comparator 202 in accordance with the waveform 212, and waveform 215 is derived from the correction signal Vc generated in the comparator 202 in accordance with the waveform 213.
It can be seen from FIG. 34 that the comparator 202 in the correction unit 102 generates a correction signal Vc having a wide pulse width (i.e., the waveform 214) when the error signal Ve has a high potential (in the case of the waveform 212), and conversely, generates a correction signal Vc having a narrow pulse width (i.e., the waveform 215) when the error signal Ve has a low potential (in the case of the waveform 213).
Therefore, in generating the error signal Ve from the pulse modulated signal Vr input from the pulse modulator 100 used for a reference and the feedback signal Vs input from the power switch 103, the error handler 101 is configured so as to generate an error signal Ve lowered in potential as the waveform 213 in the case where the pulse width of a feedback signal Vs contains an error wider than or equivalent to the pulse width of a pulse modulated signal Vr used for a reference, and to generate an error signal Ve increased in potential as the waveform 212 in the case where the pulse width of a feedback signal Vs contains an error narrower than or equivalent to the pulse width of a pulse modulated signal Vr used for a reference.
The employment of the class D amplifier having the correction system of the aforementioned configuration can automatically reduce an error of the feedback signal Vs output from the power switch 103 with respect to the pulse modulated signal Vr used for a reference.
Thus, signal distortion caused by fluctuations in the supply voltage and dead time setting in the power switch 103 can be automatically corrected, which prevents the occurrence of distortion in an audio signal output from the amplifier.
However, the correction system performing correction by means of feedback in the class D amplifier disclosed in the National Publication of Translation 2001-517393 configured as described above gives rise to the following problems.
First, in order to improve the effects of correction, the signal Vi input to the “−” terminal of the comparator 202 needs to be converted to a trapezoidal waveform signal of high accuracy. However, generating a trapezoidal waveform signal with high accuracy disadvantageously requires a circuit configuration to be complicated as compared to the circuit shown in FIG. 33.
Second, a pulse modulated signal Vr and feedback signal Vs input to the error handler 101 are pulse signals. It is very difficult to normally generate an error signal Ve from such pulse signals, and remaining pulses in the error signal Ve cannot be removed completely. Such remaining pulses disadvantageously result in difficulty of obtaining sufficient effects of correction.
In the case where a remaining pulse component cannot be removed completely, the circuit operations are restricted. That is, when the pulse component is distorted in a non-linear region of the correction unit 102, distortion occurs in the error signal Ve, which prevents correction from being performed properly. Thus, it is ideal that the error signal Ve generated in the error handler 101 should not contain a pulse component reflecting the difference between low frequency components of the pulse modulated signal Vr and feedback signal Vs.
Actually, however, phase rotation of the feedback signal Vs in the error handler 101 unstabilizes loop operations, which makes it difficult to filter the error handler 101 such that a pulse component is sufficiently attenuated. On the other hand, in order to obtain sufficient effects of feedback, the error signal Ve needs to be sufficiently amplified and corrected, which contradictorily causes a remaining pulse component to be amplified at the same time.
On the aforementioned grounds, it is difficult to obtain sufficient effects of correction (reduction of distortion in an audio signal) because of remaining pulses.
With the above-described publicly-known configuration, a PWM signal of high accuracy can be obtained, and an audio signal of high quality can be obtained as an output of the amplifier by reflecting the PWM signal to an output of the power switch with high accuracy.
However, fluctuations in supply voltage supplied to the power switch disadvantageously cause distortion in an output signal. If a voltage of a certain value is always supplied to the power switch through a constant voltage circuit, distortion in an output signal may be reduced, however, the power switch consumes relatively great power, and power loss in the constant voltage circuit for supplying a voltage of a certain value to the power switch thus increases, which causes another problem in that power amplification cannot be performed on an audio signal with high efficiency and low power loss.